I just finished a compound of 15 cubes. It took me 9 - 10 months to build it, though I had some breaks and intermezzos. The final model is 23 cm (~9″) in diameter, and the smallest pieces have an edge length of 1 mm. Here is a picture:
This particular compound can be seen as a multiplication of the classical compound of 5 cubes and 3 cubes: if you replace every cube in the classical compound of five cubes with a classical compound of three according to the symmetry, then you will get this one. That is also the most straight-forward way of colouring: each classical compound of 3 cubes has a unique colour.
In Verheyen's book "Symmtry Orbits" this is one of the rigid compounds of cubes. If you leave out the ones with cyclic and dihedral symmetry (the ones based on prisms and anti-prisms) from this group, then there are only eight of these and this one was the last one of these that I hadn't built.